We describe laboratory experiments to generate x-ray photoionized plasmas of relevance to accretion-powered x-ray sources such as neutron star binaries and quasars, with significant improvements over previous work. A key quantity is referenced, namely the photoionization parameter, defined as ξ=4πF/ne where F is the x-ray flux and ne the electron density. This is normally meaningful in an astrophysical steady-state context, but is also commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, time-dependent. We demonstrate emission-weighted values of ξ>50 erg-cm s−1 using laser-plasma x-ray sources, with higher results at the centre of the plasma which are in the regime of interest for several astrophysical scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However useful checks on performance can often be made for a limited range of parameters. For example, we show that our use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from a blackbody source with ∼keV spectral temperature. We compare calculations from our in-house plasma modelling code with those from Cloudy and find moderately good agreement for the time evolution of both electron temperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrum with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised argon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.

We present X-ray absorption measurements near the K-edge of laser heated aluminum in a pump-probe configuration using X-rays generated in a laser wakefield accelerator. A 30 fs duration laser pulse from the HERCULESlaser system was split into two beamlines, with one used to heat a 4 mu m thickness Al foil and the second to drive a laser wakefield accelerator. The laser-heated plasma was probed at various pump-probe delays using the femtosecond duration X-rays generated by betatron oscillations of the electrons in the wakefield. We observe an apparent blue-shift of the K-edge occurring on a sub-picosecond timescale in the transmission spectra.

Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilize betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.

All three parts of an atomic physics, atomic kinetics and lineshape code, ALICE, are described. Examples of the code being used to model the emissivity and opacity of plasmas are discussed and interesting features of the code which build on the existing corpus of models are shown throughout.

In this paper we report the experimental implementation of a theoretically proposed technique for creating a photoionized plasma in the laboratory using x-ray line radiation. Using a Sn laser plasma to irradiate an Ar gas target, the photoionization parameter, ξ=4πF/N_{e}, reached values of order 50ergcms^{-1}, where F is the radiation flux in ergcm^{-2}s^{-1}. The significance of this is that this technique allows us to mimic effective spectral radiation temperatures in excess of 1 keV. We show that our plasma starts to be collisionally dominated before the peak of the x-ray drive. However, the technique is extendable to higher-energy laser systems to create plasmas with parameters relevant to benchmarking codes used to model astrophysical objects.

X-ray emission spectroscopy has been used to study hot dense plasmas produced using high power laser irradiation of dot samples buried in low Z foils of plastic or diamond. By combining a high contrast short pulse (picosecond timescale) laser beam operating in second harmonic with long pulse (nanosecond timescale) laser beams in third harmonic, and with pulse shaping of the long pulse beams, a range of plasma temperatures from 400eV up to 2.5keV and electron densities from 5e22 up to 1e24/cc have been accessed. Examples are given of measurements of dense plasma effects such as ionization potential depression and line-broadening from the K-shell emission spectra of a range of low Z elements and mixtures and compared to model prediction. Detailed spectra from measurements of the L-shell emission from mid-Z elements are also presented for an example spectrum of germanium. These data are at conditions found in stellar interiors and in particular in the radiative zone of the sun. The plasma conditions are inferred from comparison of the measured spectra to detailed modeling using atomic kinetics and spectral synthesis codes.

K shell X-ray emission measurements have been used to diagnose plasma conditions in short-pulse heated buried microdot targets on the Orion high power laser. These experiments have been used to validate simulations of short pulse laser-solid interaction that combine hybrid PIC modelling of the laser absorption with radiation-hydrodynamics simulations including an electron transport model. Comparison of these simulations with streaked K shell spectroscopy show the importance of including radial gradients in fitting the spectra. An example is presented of the emission of sulphur from a 50µm diameter microdot sample buried in a plastic foil. Previously agreement between simulation and experiment was obtained only by treating the absorbed energy, electron temperature and beam divergence as fitting parameters. The good agreement obtained in this work used the measured laser energy and laser pulse length and calculated the laser-solid target interaction from first principles.

We have utilized a newly developed high-resolution X-ray spectrometer to measure the shapes of spectral lines produced from laser-irradiated targets on the Orion laser facility in the United Kingdom. We present measurements of the He-β spectra of chlorine and chromium from targets irradiated by either a long-pulse or a short-pulse laser beam. The experimental conditions provide a spread in plasma density ranging from about 1019 to about 1024 cm−3. We present spectral calculations that show that the relative intensities of the Li-like satellite lines can be used to infer the density in the lower range, especially if the lithiumlike satellite lines are well resolved. In addition, we use the Stark-broadened width of the He-β line to infer densities above about 1022 cm−3. In the case of a short-pulse irradiated chromium foil, we find that the He-like chromium is produced at a density of almost 8 g/cm3, i.e., solid density. In addition, we can infer the electron temperature from the observation of dielectronic recombination satellite lines.

Received 12 March 2016DOI:https://doi.org/10.1103/PhysRevLett.116.159502© 2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFast ignitionPlasma microinstabilitiesTechniquesParticle-in-cell methodsPlasma Physics

new ‘photon–photon collider’, which may enable elusive Breit–Wheeler pair production in an optics laboratory setting, is predicted. Using this concept, it is potentially possible to produce 10 5 Breit–Wheeler electron–positron pairs by firing a gamma-ray beam into a high-temperature radiation field of a laser-heated hohlraum cavity.

We estimate the number of electron-positron pairs which will be produced during the burning of a Deuterium-Tritium (DT) plasma in conditions that are anticipated will be achieved at the National Ignition Facility. In particular we consider, for the first time, the effect of including the gamma photons produced in a low probability channel of the DT reaction.It is found that non-thermal effects driven by the fusion products are the dominant method of pair production, and lead to a number density of positrons within the capsule in excess of 3 x 10(17) cm(-3). The positrons are predominately produced by the Bethe-Heitler process and destroyed by two photon annihilation. (C) 2014 Elsevier B.V. All rights reserved.

In a previous paper a method of calculating atomic physics data, energy levels and photo- and collisional-excitation rates using a restricted set of Slater determinants was demonstrated. This paper extends that work to demonstrate the method of calculating a generalisation of coefficients of fractional parentage using a restricted, and already known, set of Slater determinants. These coefficients can be used in photoionisation, collisional ionisation and autoionisation calculations, and so allow the methods to be extended to allow all of the data required by an atomic kinetic code to be calculated using this scheme.

We investigate the mechanism by which relativistic electron bunches created at the surface of a target irradiated by a very short and intense laser pulse transfer energy to the deeper parts of the target. In existing theories, the dominant heating mechanism is that of resistive heating by the neutralizing return current. In addition to this, we find that large amplitude plasma waves are induced in the plasma in the wake of relativistic electron bunches. The subsequent collisional damping of these waves represents a source of heating that can exceed the resistive heating rate. As a result, solid targets heat significantly faster than has been previously considered. A new hybrid model, capable of reproducing these results, is described.

In this paper we present a new result involving the properties of the Clebsch–Gordan coefficients well-used in atomic physics. We also demonstrate a practical use of this result: a new and simple method of calculation of unit tensor operators of multi-electron atoms while working within the Slater–Condon scheme. This results in a significant increase in the ease of implementation of multi-electron atomic physics models, and we discuss the computational complexity of the resulting algorithm.

Experiments have been performed in which fs-timescale laser pulses, focused to an intensity similar to 10(16) W cm(-2), are able to directly create and interact with solid density plasma (1). We have performed one-dimensional simulations of the experiments with a kinetic model which solves Maxwell's equations coupled to the Fokker-Planck equation enabling us to self-consistently model the non-local heat flow and absorption process. We find that the heat-flux is magnetized by the laser field and is inhibited relative to the Spitzer value. (C) 2012 Elsevier B.V. All rights reserved.

In ICF experiments recently undertaken at the National Ignition Facility (NIF), the high-Z dopant in the capsule ablator material has been changed from Germanium to Silicon for some experiments [1]. In this paper two issues involving the modelling of Silicon in the ablator of an ICF capsule will be addressed. Firstly, we consider the differences between LTE and non-LTE modelling of Silicon in the prediction of the opacity and of the mean charge state. Secondly, the necessary size of the photon groups (or ‘bins’) used in the radiation transport modelling will be discussed, and radiative transport calculations with varying bin sizes will be presented in order to support the conclusions.

Thomson scattering is well used as a diagnostic in many areas of high energy density physics. In this paper, we quantitatively demonstrate the practicality of using Thomson scattering as a diagnostic of short-pulse laser-plasma experiments in the regime, where the plasmas probed are at solid density and have temperatures of many hundreds of eV using a backlighter produced with an optical laser. This method allows a diagnosis both spatially and temporally of the density and temperature distributions in high energy density laser-plasma interactions which is independent from, and would act as a useful complement to, the existing spectroscopic methods.

We present the conceptual design of a new experiment to reproduce the atomic kinetics of astrophysical photoionised plasmas in the laboratory. In particular the problems of the high densities usually found in laboratory experiments and the production of high colour temperature radiation fields are discussed and solutions presented. Following an analytic discussion, which allows one to find a combination of elements, one for the radiation source and one for the element to be photoionized, the proposed experiment is modelled using existing experimental data for the radiation source and a collisional-radiative model to calculate the photoionisation history. The results indicate that this approach is feasible with current experimental parameters and techniques.

A low runtime DCA model, ALICE, is used to model the few available photoionised plasma experiments that use both laser and pulse power devices. Modelling photoionised plasmas provides a computationally intensive problem which must therefore be solved by careful approximation emphasising the underlying physics. We will demonstrate the relative importance of the completeness of the set of configurations considered compared to the set of rates used in the model and draw conclusions as to how ALICE and other models may be improved for future use.